1. Field of the Invention
This invention relates to a method for the production of a polyelectrolyte membrane and to a fuel cell.
2. Related Art
A fuel cell has an electrolyte and a pair of electrodes separated by the electrolyte. In a fuel cell, a fuel such as hydrogen is supplied to one electrode, and an oxidizing agent such as oxygen is supplied to the other electrode. This will convert the chemical energy involving oxidation of the fuel to electric energy. Hydrogen ion (i.e., proton) permeates through the electrolyte while the reaction gases (i.e., hydrogen and oxygen) does not permeate through the electrolyte. Typically, a fuel-cell stack has a plurality of fuel cells, and each of the cells has an electrolyte and a pair of electrodes separated by the electrolyte.
As electrolytes for fuel cells, solids such as polyelectrolyte membranes or liquids such as phosphoric acid are used. Among these, the polyelectrolyte membranes have received attention as the electrolytes for fuel cells in recent years. For example, perfluorosulfonic acid polymers and complexes between basic polymers and strong acids are used as materials for the polyelectrolyte membranes.
The perfluorosulfonic acid polymer, typically, has a structure in which the side chain having a sulfonic acid group (e.g., a side chain having a sulfonic acid group bound to a perfluoroalkylene group) is bound to a perfluorocarbon skeleton (e.g., a copolymer of tetrafluoroethylene and trifluorovinyl). Since the sulfonic acid group can turn into an anion through the dissociation of its hydrogen ion, it shows proton conductivity.
The polyelectrolyte membranes comprising complexes of basic polymers and strong acids have been developed. In International Publication WO96/13872 and its equivalent U.S. Pat. No. 5,525,436, there is disclosed a method for producing a proton conductive polyelectrolyte membrane by immersing a basic polymer such as a polybenzimidazole in a strong acid such as phosphoric acid or sulfuric acid. The fuel cell employing such a polyelectrolyte membrane has the advantage that it can be operated at 100° C. or above.
In J. Electrochem. Soc., Vol. 142, No. 7, 1995, ppL 121-L123, it is described that when a polybenzimidazole is immersed in 11 M phosphoric acid for at least 16 h, the polybenzimidazole will be impregnated with five molecules of phosphoric acid per unit.
Further, in International Publication WO97/37396 and its equivalent U.S. Pat. No. 5,716,727, there is described a method for producing a polyelectrolyte membrane by obtaining a solution of polybenzimidazole dissolved in trifluoroacetic acid, next by adding phosphoric acid to the solution, and subsequently by removing the solvent.
All the disclosures of WO96/13872, J. Electrochem. Soc., Vol. 142, No. 7, 1995, ppL 121-L123, and WO97/37396 are incorporated into the present specification by reference.
Where the complexes between basic polymers and strong acids are to be put into practical use as the polyelectrolyte membranes for fuel cells, further improvements on their proton conduction are needed.
In addition, where such polyelectrolyte membranes are manufactured, it is required from the standpoint of their production process that the times of immersion of the basic polymers in the strong acids be brief. In J. Electrochem. Soc., Vol. 142, No. 7, 1995, ppL 121-L123, a polybenzimidazole is immersed in phosphoric acid for at least 16 h. This is too time-consuming and the production process will prove to be inefficient.